How to select the right Projection Screen
What you need to know about proper selection of front and rear projection screens

Most people and businesses greatly underestimate the importance of a projection screen when planning presentations and events. The projection screen is the most important and yet the least considered essential for planning audio visual presentations and events. Companies that do not understand the basics for effective audio visual presentations will spend thousands of dollars on the latest data projector and then ruin the effect by showing the presentation projecting it onto an unsuitable screen or even a wall. This is foolish because the end result is to counter-act the benefits of buying a quality projector.

Powerful content and good imagery is the key to successful presentations and events but without an eye-catching screen the content has no chance of making an impact. So don’t just concentrate on how many ANSI lumens your projector has and consider that your audience looks at the image on the screen and the projector cannot do the job with the right screen. 

Here are the things that you need to consider about projection screens before planning any AV presentation or event.

Use a projection screen for enhanced image quality!

The purpose of a projection screen is to enhance the quality images or video to be displayed by reproducing the image without losing quality and distributing the light of the projection image towards the audience.

How does a projection screen actually work?

Projection screens maximize the light from the projector by directing and reflecting more light back to the audience within the concentrated viewing angle. Projection screens are manufactured using high-tech fabrics that control, reflect and efficiently distribute the projected image directly to the audience. The screen does this by reflecting optimum image quality and clarity. Optimum image quality can only be achieved if the projector and projection screen are properly matched in the presentation room.

Facts to consider before selecting a projection screen

The type and size and fabric of a projection screen required for an event or presentation will bet determined by

• the ambient light
• audience size
• room size and configuration
• viewing angle
• the type of image to be projected.

Making an incorrect choice of screen size or screen fabric will affect the quality of the presentation and may mean that people may not be able to view the image properly.

Choosing a screen with the right aspect ratio

The aspect ratio of the projector is the key element in selection a screen with the aspect ratio. If your projector is natively 4:3 match this with a 4:3 screen. This will give you the best effect filling all available screen space and optimising the projected image. So when you know the aspect ration of the projector then you can consider choosing a screen with the correct ratio  of 1:1 or 4:3, or 16:9.

Screen Fabrics can be important in certain applications.

Matte White Diffusion Screen Surface - This is the most widely used projection screen surface. It is suitable for most applications because it evenly distributes light over a wide viewing area. In controlled light conditions the colours remain bright and life-like, with no shifts in hue but quality may be effected where light conditions are not controllable.

Data Lux Reflective screen fabric - This is a special projection screen fabric specially developed to deliver a higher reflection value for projection applications requiring a wider viewing angle. This fabric well suited for video and data projection its most beneficial use is with ceiling mounted projectors where light and viewing angels affect the image quality.

Retro-reflective screen fabric - is recommended for smaller presentations with fewer people in the room where the projector is at table height. This screen fabric reflects the projected image light back towards the projector. Because the light rays are bounced back along the exact path back they came from, it is not recommended for ceiling mounted projectors or for offset projection angles.

The Viewing Angle can cause of unexpected disappointment. Unless screens have specifically engineered optical characteristics, then the line of peak brightness is a line you draw from the projector lens, running through the centre of the screen.

Rear Projection - Rear projection screens are considerably more expensive than front projection screens. They are constructed of a single piece acrylic sheet that is durable and light weight. All optical properties in a rear projection screen are manufactured inside the screen surface which is anti-glare and anti-reflective enabling it to be used even in daylight. The projected image is uniform  across the screen surface without any hot spots and the screen resolution is infinite, limited only by the projector technical values. The are capable of producing extraordinary quality of image even under very high ambient light conditions.

Rear Projection screens also have the benefit that people are les likely to be able to walk in between the projector and the screen.

Here are some of the basic facts relevant to projecting the right image;

• Contrast - Contrast is the key to a brighter image. The word ‘bright’ itself is misleading when applied to projection. Yes you do need a threshold level of brightness. But without proper black levels the image is a wash-out. Front screens cannot distinguish between projected and other light; so ambient light ‘pollutes’ the black - ruining the contrast. Rear projection screens do not have this problem. Without contrast there’s no image, no matter how ‘bright’ it is.

• Screen Brightness - has a fixed relationship to image area. A screen of 2m squared is 50% dimmer than of one of 1m squared. Screen brightness is also relative to gain.

• Image Area  - increases drastically relative to its diagonal size. A small increase in the diagonal size is a big increase in image area. A 100” screen has four times the image of a 50” screen - and thus only 25% its brightness.

• Light - Our eyes don’t register light on a normal scale. We can only register a difference when light levels double or halve. This is why the apparently super-bright screen in the showroom can look so weak in a shop window - and why a projector of 2,000 ANSI lumens seems hardly brighter than one of 1,000 ANSI lumens.

• Screens behind glass present special problems. Glass is reflective. The greater the angle the light strikes glass, the more reflective it becomes. After approximately 45° it’s more like a mirror. A normal office has approximately 500 lux ambient light; outside on a bright sunny day it can be greater than 120,000 lux (i.e. over 240 times brighter). So in some locations the brightest of projector screens might not be viewable at certain times of the day (unless the glass is treated). It needs planning and a sense of reality; each installation is different.

 Projection Screen Terminology

Gain - A relative measure of a screen's reflectivity.
Contrast - The ability to accurately reproduce and differentiate light and dark characters and backgrounds, or light and dark areas of an image.
Ambient Light Rejection Properties - The ability to perform well under normal to adverse lighting conditions in the audience area.
Resolution - The clarity of the projected image.
Uniformity - The screen's performance when viewed from various points off the projection axis (both horizontally and vertically), and when the brightness of the centre of the image is compared to the corners.
Projection Format - The height and width of the projected image determines the screen's size and shape (AV, NTSC, HDTV, Widescreen, CinemaScope, overhead, slide or motion pictures). 
 

By John Corrick
http://www.ezehire.com/au/projectionscreenhire

11th July 2009

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